High density polyethylene (HDPE) is used in sheet extrusion/thermoforming operations to produce a large variety of large parts such as truck bed liners, “port-a-potties” or portable toilets, and “dunnage trays” for holding and transporting large industrial parts such as transmissions, etc. The intended use of such products dictate that the fabricated part or container meet certain minimum requirements, such as stiffness, impact resistance, top load, Environmental Stress Crack Resistance (ESCR), and chemical resistance. In addition, the manufacturers of such parts desire ease of processability including thermoformability. Accordingly, the polymers chosen for use in extruded sheet and thermoforming applications require a balance of rheological properties. The ideal polymers will provide suitable kinematics for improved extrudability, and adequate sag or drape resistance while simultaneously allowing adequate extensibility for uniform and consistent final part thickness. This will ideally be accomplished without sacrificing any of the desired solid state performance properties of the polymer.
In the development of resin there is typically a trade off between characteristics, such as resistance to slow crack growth and rupture (measured, for instance, by Environmental Stress Crack Resistance or ESCR), stiffness (measured, for instance, by flexural or secant modulus), and toughness (measured by an impact test), and processability (measured, for instance, by shear and extensional flows). Typically the higher the polyethylene molecular weight, the better the solid state properties like ESCR. However, increasing the molecular weight will tend to decrease processability making profile and sheet extrusion and thermoforming more difficult.
High-molecular-weight (HMW) ethylene homopolymers and copolymers typically exhibit improved strength and mechanical properties, including high tensile strength, impact strength and puncture resistance. However, attendant with such increases are difficulties in processability and extrudability of these HMW resins. One approach to solve this problem has been to broaden the molecular weight distribution (MWD) of the HMW polyethylene. One method to achieve this is by catalyst selection, for instance, it is known that chromium catalysts tend to produce a product with broader molecular weight distribution than either traditional Ziegler-Natta (Z-N) or the newer metallocene-based catalyst systems.
Chromium catalysts are well known catalysts for olefin polymerization and are useful in preparing HMW HDPE. In these catalysts, a chromium compound, such as chromium oxide, is supported on a support of one or more inorganic oxides such as silica, alumina, zirconia or thoria, and activated by heating in a non-reducing atmosphere. U.S. Pat. No. 2,825,721 describes chromium catalysts and methods of making the catalysts. It is also known to increase polymer melt index by using a silica-titania support as disclosed, for example, in U.S. Pat. No. 3,887,494. Numerous activation procedures have been described in the prior art for optimizing catalyst performance and resultant ethylene polymer characteristics, such as U.S. Pat. No. 4,981,831, U.S. Pat. No. 5,093,300, U.S. Pat. No. 5,895,770, U.S. Pat. No. 6,150,572, U.S. Pat. No. 6,201,077, U.S. Pat. No. 6,204,346, U.S. Pat. No. 6,214,947, U.S. Pat. No. 6,359,085 and U.S. Pat. No. 6,569,960, US2001/0004663 and US2001/0007894, EP1038886A1, EP0882740A1, EP0882743A1, EP0905148, and WO00/14129 and WO2005/052012. While these known techniques help in optimizing the resulting polymer characteristics, it has been observed that the current resins made via the gas phase process still exhibit a higher degree of sag or drape than resins made using the slurry loop process. This higher sag requires processors using gas phase resins to change tooling or process conditions relative to other resins, and as result, resins from the gas phase process have not been widely accepted in the industrial thermoforming market.
Another method used to overcome the processing difficulties associated with HMW polyethylene has been to increase the MWD of the polymer by providing a blend of a HMW polymer with a low-molecular-weight (LMW) polymer. The goal of such a formulation is to retain the excellent mechanical properties of the high molecular weight polyethylene, while also providing improvements in processability, resulting from the improved extrudability of the lower molecular weight component. For example, U.S. Pat. No. 6,458,911 and US2002/0042472A1 disclose a bimodal ethylene polymer film resin comprising a polymer blend, of a LMW component and a HMW component. The blends are said to be capable of being formed into high strength thin films. These processes add unwanted complexity and expense to the process however, and so it would be desirable to have a single resin which would adequately deliver the desired combination of properties.
Accordingly, it is desired to develop a gas-phase HMW HDPE resin having improved sag or drape resistance without unduly limiting the extensibility.
The invention provides such a composition, a process to make such compositions and products made by the compositions.
More particularly, the present invention relates to the use of oxygen tailoring to increase the melt strength of chromium catalyzed HDPE without decreasing the extensibility or elongational viscosity properties of the resin to such a point that the HDPE no longer has enough extensibility to make various part sizes and types.
Oxygen tailoring is a known process whereby molten polyethylene resin is exposed to low levels of oxygen at normal polymer melt temperatures which allows for the limited coupling of polymer chains. However, the use of oxygen tailoring has heretofore primarily been focused on bimodal HDPE, and primarily for blown film applications (see for example U.S. Pat. No. 5,728,335, U.S. Pat. No. 6,454,976, EP0936049, WO2004/005357, WO2005/061561, US2004/0039131A1 and US2005/0012235A1).
It has surprisingly been found that the oxygen tailoring process can also be used to reduce the sag in extruded sheets made from HDPE. Without intending to be bound by theory, it is believed that the oxygen tailoring introduces low levels of long chain branching in the resin. This long chain branching increases the melt strength of the resin, but it is believed that the levels of long chain branching are low enough so as not to cause significant changes or unduly limit neither the extensional viscosity nor the solid state performance of the resin. This effect is particularly noticed with HDPE made using chromium based catalysts.
Accordingly in a first embodiment, the present invention comprises a process for making an extruded sheet comprising the steps of conveying an HDPE resin through an extruder, wherein the extruder comprises a feed zone, a first melt zone downstream of the feed zone, a second melt zone downstream of the first melt zone and a third melt zone downstream of the second melt zone; contacting the HDPE resin with a gaseous medium comprising oxygen in the second melt zone, under conditions sufficient to promote at least some long chain branching, thereby producing a modified HDPE resin; contacting the modified HDPE resin with at least one antioxidant (which may be a primary antioxidant, a secondary antioxidant, or a combination of the two antioxidants, with or without other adjuvants) in the third melt zone; passing the resin which has been contacted with the antioxidant through a die to form sheet having a thickness in the range of 0.25 mm to 25 mm. It should be readily understood by a person of ordinary skill in the art that the resin which has been contacted with the antioxidant may first be formed into pellets, which may thereafter be extruded into sheet.
Sheets made in such a fashion will be characterized as having less sag, than a corresponding sheet of the same thickness produced using an unmodified HMW HDPE chrome catalyst in the gas phase process. Sag can be represented by the drooping (or sag) of resin in the thermoforming operation at the completion of the heating cycle. Lower drooping (or sag) levels are desired. It has been observed that the observed sag is related to either the viscosity at 10−2 sec−1 in dynamic mechanical spectroscopy (DMS) test, which measures viscosity as a function of oscillation frequency, or by the ratio of the viscosity at 10−4 sec−1, as measured in a creep test, to the viscosity at 102 sec−1 shear rates. Resins with higher viscosities and/or higher viscosity ratios are expected to have higher resistance to sag.